Crystalline molecular sieves have a 3-dimensional, microporous frameworks having tetrahedrally coordinated cation [TO4]. Generally, frameworks comprising oxygen tetrahedra of aluminum and silicon cations leads to the formation of microporous aluminosilicate framework commonly known as zeolites. On the other hand, 3-dimensional microporous aluminophosphate (AlPOs) frameworks classified as zeo-type molecular sieves are composed of oxygen tetrahedra of Al and P cations whereas silicoaluminophosphate (SAPOs) type molecular sieves composed of oxygen tetrahedra of Si, Al and P cations.
Molecular sieves are typically described in terms of the size of pore window which is based on the number of T atoms present in the pore window. Typically they are classified as small, medium and large pore molecular sieves based on their pore opening. The small pore molecular sieves have pore size in between 0.4-0.5 nm. Medium pore molecular sieves have pore size in between 0.5-0.6 nm whereas large pore molecular sieves have pore opening of 0.6-0.8 nm (R. Szostak, Molecular Sieves: Principles of synthesis and Identification, 2nd edition, Blackie Academic and Professional, London, 1998).
Wide spread applications of crystalline molecular sieves in the field of petroleum processing, petrochemical, fine chemical has led to sustained research effort, both in industry and academia, for their discovery. This has resulted into synthesis of new frameworks such as ITQ-13 (Corma, et. al., Angew. Chem. Int. Ed. 42, 1156-1159 (2003)), ITQ-12 (Barrett et. al. Chem. Commun. (2003)), SSZ-53, -59 (Burton et. al. Chemistry: a Eur. Journal 9, 5737-5748 (2003), Chemistry: a Eur. Journal 9, 5737-5748 (2003)) in recent times.
Molecular sieves are usually synthesized under hydrothermal conditions from a reactive gel comprising of aluminum, silica and/or phosphorous sources in the presence of an organic structure directing agent, such as an organic nitrogen compound in the temperature range of 100-200° C. Commonly used nitrogen compounds are amines, diamines and quaternary ammonium salts.
Such syntheses are also carried out under solvothermal conditions employing non-aqueous solvents such as glycols. The use of mineralizing agents is also practiced in molecular sieve synthesis. For example, patent publication EP-A-337,479 discloses the use of hydrogen fluoride in water at low pH to mineralize silica in glass for the synthesis of ZSM-5. The use of fluoride medium is also depicted in U.S. Pat. No. 6,793,901 for synthesis of aluminophosphate or silicoaluminophosphate molecular sieves having the CHA framework. The use of fluoride media is also reported to lead to the formation of large zeolite crystals (Berger et. al. Microporous and Mesoporous Materials 83, (1-3), 2005, 333-344).
Some of the zeolites cited above have been synthesized in a fluoride medium in which the mobilizing agent is not the usual hydroxide ion but a fluoride ion in accordance with a process initially in U.S. Pat. No. 4,073,865. One advantage of such fluorine-containing reaction systems is to allow the production of purely siliceous zeolites containing fewer defects than zeolites obtained in a traditional OH− medium (J M Chezeau et al, Zeolites 1991, 11, 598). A further decisive advantage of using fluorine-containing reaction media is to allow the production of novel framework topologies containing double cycles of four tetrahedra, as is the case with ITQ-7, ITQ-13, ITQ-12 and ITQ-17 zeolites.
Recently, the use of fluoride medium has led to crystallization of novel large pore aluminophosphate based molecular sieve of SFO type (Morris et al. Chem. Mater. 2004, 16, 2844).
More recently, the crystallization of EMM-8 phase (United States patent publication 2006/0074267) has been disclosed from fluoride free medium. Such framework has been claimed to be isostructural to SFO type framework on calcination.
The crystalline molecular sieve composition, BPC-1, disclosed in the present invention exhibits unique X-ray diffraction pattern with four peaks in the range of 2 theta 5-10° and differs with that of EMM-8. Thus its structural framework is primarily different than that of EMM-8.
Generally the crystallization of molecular sieves is performed under hydrothermal conditions in the temperature range of 100-200° C. which usually requires prolonged crystallization time for phase formation. This sometimes leads to the formation of thermodynamically stable dense phases such as tridymite, cristobalite, berlinite, quartz as impure phases. This is due to the metastable nature of zeolitic framework under crystallization conditions. Furthermore, conventional hydrothermal approach is often found to be energy intensive.
The microwave-assisted synthesis of molecular sieves is a relatively new area of research (Komarneni, et. al. Mater. Res. Bull. 1992, 27, 1393; Ionics 1995, 21, 95). It offers many distinct advantages over conventional synthesis. They include rapid heating to crystallization temperature due to volumetric heating, resulting in homogeneous nucleation, and fast supersaturation by the rapid dissolution of precipitated gels and eventually a shorter crystallization time compared to conventional autoclave heating. It is also energy efficient and economical.
This method has been successfully applied for the synthesis of several types of zeolites namely zeolite A, Y, ZSM-5, MCM-41, metal substituted aluminophosphate and gallophosphate. It has also been successfully applied for the synthesis of mesostructured thiogermanates/germanium sulfides. A rapid synthesis of titanium substituted MCM-41 molecular sieve has also been reported using microwave assisted approach. Recently, a rapid synthesis of SBA-15 and Ti-, and Zr-SBA-15 framework under microwave-hydrothermal conditions has been reported (Cundy, C. S. Coll. Czech. Chem. Commun. 1998, 63, 1699, Oberender et. al. Mat. Res. Symp. Proc. 1999, 547, 433. Kang, et. al. Catal. Lett. 1999, 59, 45. Newalkar, et. al. Chem. Commun. 2000, 2389, Chem. Mater. 2001, 13, 552).
The use of microwave-hydrothermal conditions in our research has led to the invention of novel microporous aluminophosphate framework (designated as BPC-1) in the presence of fluoride ions. The crystallized framework also appears to be isostructural to SFO type framework upon calcination.